The genomic clone containing the putative promoter region of rat malic enzyme gene has been obtained and sequenced. None of the expected consensus sequences (TATA and CAAT box) were found. Instead, this region is G/C rich and contains in direct repeat six CCGCCC hexanucleotides upstream of the transcription site. Several deletion mutations at this region were analyzed for a promoter activity by calcium phosphate mediated transfection into Chinese hamster ovary cells. Preliminary results show that -165 base pairs can direct the transient expression of the structural part of the CAT gene, while -40 and -85 base pairs are insufficient. The chromatin structure of the malic enzyme gene has been analyzed in different thyroidal states. No differences in methylation patterns were detected after thyroid hormone treatment. It appears that a number of hypersensitive sites at the 5' and 3' end of this gene do not depend on thyroid hormone, but some of these sites are substantially stronger in hyperthyroid animals. However, a great majority of the malic enzyme gene does not demonstrate hypersensitivity sites. This suggests heterogeneity of malic enzyme gene activity in hepatocytes, resulting from a different functional capacity of individual hepatocytes to synthesize malic enzyme. Presently, we are investigating this possibility.